Method for making high tenacity regenerated cellulose staple fiber



Nov. 5, 1963 E. KLEIN EI'AL 3,109,693

METHOD FOR mm; HIGH 'rsmcm REGENERATED CELLULOSE STAPLEFIBER Filed April2, 1962 VISCOSE Spinning in acid bath in presence of formaldehyde F ilaments 2 20% regenerated Stretching in bath at 60-95% containing 0.5%acid by weight and 0. 2%formaldeh yde by weight Streiched fi lamenis atleast 90%regeneraied 3,109,698 METHOD FOR MAKING HIGH TENACITY RE-GENERATED CELLULOSE STAPLE FIBER Elias Klein, Harold Wise', and WilliamC. Richardson, Mobile, Ala., assignors to Courtaulds North America Inc.,a corporation of Alabama Filed Apr. 2, 1962, Ser. No. 184,510 6 Claims.(Cl. 18-54) This application relates to the manufacture of high tenacityrayon filaments and in particular to the manufacture of high tenacityrayon staple fiber.

In recent years there has appeared on the market a new type of rayonfiber characterized by a high degree of crystallinity, a high tenacity,a more or less circular, non-crenulated cross section and the absence ofany appreciable skin-core structure. In general such fibers are made byextruding a substantially unripened viscose, the cellulose of which hasa high degree of polymerization, into an aqueous bath relatively low inregenerative power and stretching the filaments while they remainsubstantially unregenerated.

Various specific techniques have been suggested for carrying out thisgeneral process. In particular it has been suggested that formaldehydebe incorporated in the spinning bath to retard regeneration of thefilaments and that the filaments be stretched in a subsequent hot bathcontaining, say, 14% acid. This process, when operated under optimumconditions, and particularly when used for the manufacture of continuousfilaments can give excellent results. However, it is open to severaldrawbacks.

Thus, for example, in the manufacture of staple fiber it is necessarythat the filaments be substantially entirely regenerated before they arecut. Cutting necessarily involves relaxation and if the filaments,having been stretched, are allowed to relax before they are regenerated,the effect of the stretch is largely lost. The hot acid stretch bathused in prior processes, while it is regenerative for filaments spun inhigh zinc baths, is not very regenerative for filaments spun in thepresence of formaldehyde. The cellulose xanthate methylol ester formedas the result of such spinning must be hydrolysed in regeneration andits hydrolysis is suppressed by the presence of H+ ions.

Because acid stretch baths are ineflicient for regeneration it isnecessary either to greatly extend the travel of the filaments throughthe bath or to add a subsequent separate regeneration step. Neither ofthese alternatives is very attractive, particularly when staple fiber isheirs; made, since it is ordinarily desirable to cut the staple as soonas possible and conduct the washing, desulfurization and like aftertreatments on the cut fiber.

In addition, the hot dilute acid bath, which always contains someformaldehyde carried over by the filaments, favors the formation ofvarious undesirable byproducts, notably trithiane, which tend todiscolor the threads and are difiicult to remove from the bath liquor.

Finally the temperature required in the bath, which is normally justbelow boiling, demands a considerable heat expenditure and gives rise tofume problems through vaporization of formaldehyde.

In accordance with the invention, these and other problems are overcomeby maintaining the stretch bath acid concentration below about 0.5% andnormally between about and about 0.4%. By maintaining the acidconcentration at this level, regeneration is accelerated, permitting thefilaments to be relaxed as soon as they are removed from the bath.Moreover, the formation of trithiane is suppressed and the temperatureof the bath is substantially reduced.

I United States Patent 0 3,109,698 Patented Nov. 5, 1963 The inventiontherefore envisions, in a method for making high tenacity rayonfilaments, wherein. viscose is extruded into an aqueous acid coagulatingbath in the presence of small quantities of formaldehyde to formfilaments. the filaments are removed from said coagulating bath whilestill not more than about 20% regenerated, and are then stretched in aregenerating bath, the improvement which comprises maintaining the acidconcentration of said regenerating bath at less than about 0.5% byweight.

THE VISCOSE The viscose used in the present process is preferably madefrom pulp whose cellulose has a high. degree of polymerization, (DP),say over 600. Of course, cellulose having lower DP, say 300-600, can beused, at a sacrifice of tenacity in the product. The viscose can bemanufactured in conventional manner, but preferably the alkali celluloseis not aged, and the viscose is preferably spun without substantialageing. It may contain, for example, from 2 to 7% cellulose, and from 2to 9% NaOl-l. It may have a salt (NaCl) figure of 12 to 23, a gammanumber of not less than 60, normally to 90, or even higher, and aviscosity (ball fall) of 30 to 400 seconds, which will be determined bythe cellulose content and DP of the viscose.

SPINNING BATH The viscose is preferably spun into a spinning bath havingrelatively low regenerative power. Normally it will contain from say 3to 9% H 50 The sodium sulfate concentration is normally allowed to reachthe equilibrium value determined by the acid concentration of the bathand the soda concentration of the viscose. Usually it will be from say 4to 18%. Sinc sulfate is preferably omitted or, if present, is maintainedbelow about 0.2%.

The bath will also contain a small quantity, say between about 0.5 andabout 2.0%, preferably between about 0.5 and about 1.0 of formaldehyde,to retard regeneration. If desired, formaldehyde may be added to theviscose. However, if this is done it is recommended that the tech niquesdescribed in the copending application of Klein et 111., Serial No.192,322, filed May 4, 1962, and assigned to the assignee of the presentapplication, be employed, i.e. that the formaldehyde be in substantiallymonomeric form prior to injection and that it be injected just prior tospinning.

The temperature of the spin bath will normally range from say 20 C. to35 C.

The spinning speed will normally be determined by the viscosity of theviscose. It can range from say 20 m./m. to 50 m./m. The extrusion ratiois preferably around 0.5, say from about 0.2 to 0.7. The residence timeof the filaments in the spinning bath will be determined by the spinningspeed, extrusion ratio and stretch to be applied. In most instances itwill be between about 1 and about 4 seconds.

STRETCH CONDITIONS As noted above, the viscose is preferably spun at agamma value of about 75 to about 85. As the coagulated filaments areremoved from the spinning bath, their gamma value is still above about65, or not less than of its value prior to spinning. Thus the filamentsare not more than say 20% regenerated, are plastic and are capable ofbeing stretched to a large degree. In accordance with the invention, thefilaments are now passed through an aqueous stretch bath containing notmore than about 0.5% H 80 and, while in said bath, are stretched by anamount which may range from say to say As noted, the acid concentrationof the bath is maintained at below 0.5%. There is really no lower limitto the concentration, except that the pH must be 7 or less in order toprevent reformation of the sodium cellulose xanthate. Conveniently,fresh water is constantly added to the bath and a small amount of bathiscontinually removed. The small ar nounteof acid carried over by thethread is relied upon to maintain the bath on the acid- Under normalcircumstances the acid concentraside. tion will be less than about 0.2%,say to 0.1%.

The foregoing figures are, of course, given in terms of sulfuric acid.It is possible to operate with other acids, and in this case the valuesdefined above may be converted to the equivalents in terms of hydrogenions formed.

It is also important to maintain the formaldehyde content of the bathbelow thatwhich would retard regeneration, bearing in mind that someformaldehyde is carried into the bath with the filaments. In general theformaldehyde concentration should be kept below about 0.2%, usuallybelow about 0.1% Under the spinning conditions described, these limitsare normally automatically reached when the acid concentration in thestretch bath is maintained constant by continually withdrawing a smallquantity of the bath and replacing it with fresh water.

As noted, the temperature of the stretch bath may be maintainedconsiderably below that normally required. For example, a temperature of60 to 80 C. is in general adequate, though higher temperatures up to say95 C. may be used.

The filaments are maintained in the stretch bath until they are at least90% regenerated, i.e. until their gamma values have decreased to say 7to 8.

After removal from the bath, the filaments may be cut up into staple, ifdesired, or processed as continuous filaments in accordance withconventional practice.

The invention will be further described with reference to the followingspecific examples which are given for purposes of illustration only andare not to be taken as in any way limiting the scope of the appendedclaims.

Example I A viscose containing 5.0% cellulose, 6.5% NaOH, and having 52%carbon disulfide based on cellulose (7:79) was spun at a salt figure of19.0 and a ball fall viscosity of 130 seconds into an aqueous spinningbath containing 6.15 H 80 12% Na,S0 and 0.8% HCHO. The bath temperaturewas about 27 C. After a total residence time in the spinning bath ofabout 1 second the filaments, still less than 20% regenerated, wereremoved from the spinning bath and stretched 260% in a secondary bath.The secondary bath had a temperature of 80 C. It contained about 0.1% H80 and had a pH of somewhat less than 2. It contained about 0.1% HCHO.

After removal from the stretch bath, the filaments were cut up to formstaple fiber, washed in the usual manner and dried. The final product(1.55 denier) had a dry tenacity of 5.27 g./den., a dry elongation of6.9%, a wet tenacity of 4.20 g./ den. and a wet elongation of 7.1%

Example ll A viscose containing 3.0% cellulose, 4% NaOH, and having 52%CS on cellulose was spun at a salt figure of 20 into a bath containing6.8% H 80 6% Na SO and 0.5%

- HCHQ at 27 C. It was removed while having a gamma number of 85 andstretched 300% in the secondary bath maintained free from CH O and H SOcut into staple, washed, and dried. The final product (1.20 denier) hada dry tenacity of 5.05 g./den., a dry elongation of 4.9%, a wet tenacityof 3.52 g./den. and a wet elongation of 5.9%.

The fiber obtained in Examples I and II was white and showed no tendencytoward yellowing or other discoloration. Small quantities of trithianecould be detected in the efliuent from the bath. However, these weremuch less than normal and because of the low acid content of the bath,it was economical to throw the efiiuent away, thus avoiding the problemof separating the impurity.

What is claimed is:

1. In a method for making high tenacity rayon filaments wherein viscoseis extruded into an aqueous acid coagulating bath in the presence ofsmall quantities of formaldehyde to form filaments, the filaments areremoved from said coagulating bath while still not more than 20%regenerated and are then stretched in an aqueous stretch bath having atemperature between 60 C. and about 95 C., the improvement whichcomprises maintaining the acid concentration of said stretch bath atless than 0.5%, and the formaldehyde concentration at less than 0.2%, byweight and maintaining the filaments in said secondary bath until theyare at least 90% regenerated.

2. The method claimed in claim 1 wherein the stretch bath contains lessthan 0.1% by weight formaldehyde.

3. The method claimed in claim 1 wherein the stretch bath has atemperature of between about C. and about C. l

4. In a method for making high tenacity regenerated cellulose staplefiber wherein viscose is extruded into an aqueous acid coagulating bathin the presence of small quantities of formaldehyde to form filaments,the filaments are removed from the coagulating bath while not more than20% regenerated, are then stretched in an aqueous stretch bath having atemperature between about 60 C. and about 95 C. and are subsequently cutup to make staple, the improvement which comprises maintaining the acidconcentration of the stretch bath at less than 0.5%, and theformaldehyde concentration at less than 0.2%, by weight and maintainingthe filaments in said stretch bath until they are at least regenerated.

5. The method claimed. in claim 4 wherein the stretch bath contains lessthan 0.1% by weight formaldehyde.

6. The method claimed in claim 4 wherein the stretch bath is at atemperature between about 60 C. and about 80 C.

References Cited in the file of this patent UNITED STATES PATENTS2,452,130 Kayser Oct. 26, 1948

1. IN A METHOD FOR MAKING HIGH TENACITY RAYON FILAMENTS WHEREIN VISCOSE IS EXTRUDED INTO AN AQUEOUS ACID COAGULATING BATH IN THE PRESENCE OF SMALL QUANTITIES OF FORMALDEHYDE TO FORM FILAMENTS, THE FILAMENTS ARE REMOVED FROM SAID COAGULATING BATH WHILE STILL NOT MORE THAN 20% REGENERATED AND ARE THEN STRETCHED IN AN AQUEOUS STRETCH BATH HAVING A TEMPERATURE BETWEEN 60*C. AND ABOUT 95*C., THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE ACID CONCENTRATION OF SAID STRETCH BATH AT LESS THAN 0.5%, AND THE FORMALDEHYDE CONCENTRATION AT LESS THAN 0.2% BY WEIGHT AND MAINTAINING THE FILAMENTS IN SAID SECONDARY BATH UNTIL THEY ARE AT LEAST 90% REGENERATED. 